In this project, the role of the olivocochlear bundle (OCB) will be explored in multiple studies of peripheral auditory development. The finding the peripheral auditory function in animals reared in the absence of the OCB is decidedly pathological suggests that the OCB plays a role in cochlear development. However, an alternative possibility is that animals that lose their OCB early in life are highly susceptible to acoustic trauma. In proposed experiments, the peripheral auditory consequences of rearing animals deefferented at birth in a quiet environment will be studied, in addition to deefferented animals exposed to noise in an attempt to determine the source of the cochlear pathology associated with OCB transection. Auditory nerve pathology produced by such lesions is consistent with the malfunction, or abnormal expression, of the cochlear amplifier. Auditory nerve fibers from adults deefferented at birth exhibit elevated thresholds, relatively broad tuning, low spontaneous discharge rages and poor tip-totail rations. Peripheral auditory nonlinearity in animals lesioned as neonates will be assessed to test the hypothesis that OCB interactions with outer hair cells (OHCs) are required for the normal expression of active hypothesis that OCB interactions with outer hair cells (OHCs) are required for the normal expression of active cochlear mechanics. A separate experiment will test the hypothesis that the OCB plays a role in the refinement of exuberant synapses between inner hair cells and auditory nerve dendrites that occurs during development. While the proposed experiments will focus primarily on peripheral auditory pathology generated by early deefferentation, OCB development, per se, will continue to be explored. This will be done by determining the role of OHCs in the OCB-IHC detachment and intracochlear migration dynamic that occurs early in feline life, by destroying OHCs prior to the onset of hearing and anatomically evaluating the disposition of OCB terminals within the organ of Corti in physiologically characterized animals. Finally, the gap in our understanding of structure/function aspects of OCB development will be filled by studying its cochlear innervation histologically and ultrastructurally in physiologically characterized animals.